Although we breathe more than one cubic meter of air every hour, our lung defense mechanisms usually deal with the large quantities of particles, antigens, infectious agents and toxic gases and fumes that are present in inhaled air. The interaction of these particles with the immune system and other lung defense mechanisms results in the generation of a controlled inflammatory response which is usually protective and beneficial. In general, this process regulates itself in order to preserve the integrity of the airway and alveolar epithelial surfaces where gas exchange occurs. In some cases, however, the inflammatory response cannot be regulated and the potential for tissue injury is increased. Depending on the type of environmental exposure, genetic predisposition, and a variety of ill-defined factors, abnormally large numbers of inflammatory cells can be recruited at different sites of the respiratory system, resulting in illness or disease.
The inflammatory response to inhaled or intrinsic stimuli is characterized by a non-specific increase in the vascular permeability, the release of inflammatory and chemotactic mediators including histamine, eicosanoids, prostaglandins, cytokines and chemokines. These mediators modulate the expression and engagement of leukocyte-endothelium cell adhesion molecules allowing the recruitment of inflammatory cells present in blood.
A more specific inflammatory reaction involves the recognition and the mounting of an exacerbated, specific immune response to inhaled antigens. This reaction is involved in the development of asthma, Hypersensitivity pneumonitis (HP) and possibly sarcoidosis. Dysregulation in the repair mechanisms following lung injury may contribute to fibrosis and loss of function in asthma, pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), and chronic HP.
It was previously reported that the incidence of HP is much lower among current smokers than in non-smokers (1-4). Sarcoidosis is also less frequent in smokers than in non smokers (5, 6). The mechanisms underlying the beneficial effects of cigarette smoking on the development of HP and other inflammatory diseases are still unknown but may be linked to the immunomodulatory effect of nicotine. There are clinical observations of asthma de novo or exacerbation after smoking cessation. Proof of this is difficult to obtain and any protective effects of nicotine in the prevention or treatment of asthma are likely overwhelmed by the negative effects of tobacco smoke with its thousands of constituents.
The protective effect of smoking has also been reported in other diseases, the most studied being ulcerative colitis, an inflammatory intestinal disease (7, 8). Nicotine has been successfully used in the treatment of this disease (9, 10). Other studies have looked at the possible therapeutic value of nicotine in the treatment of Alzheimer's disease and Parkinson's disease (11, 12).
Nicotinic receptors are pentamers made up of five polypeptide subunits which act as ligand-gated ions channels. When the ligand binds to the receptor, a conformational change in the polypeptide occurs, opening a central channel that allows sodium ion to move from the extracellular fluid into the cytoplasm. Four types of subunits have been identified: α, β, γ and δ. The receptor can consist of any combination of these four types of subunits (13). Recent work has shown that alveolar macrophages (AM) can express the α-7 subunit (14), while bronchial epithelial cells express the α-3, α-5 and α-7 subunits (15), and lymphocytes the α-2, α-5, α-7, β-2 and β-4 subunits (14). Fibroblasts (16) and airway smooth muscles cells (17) also express these receptors. Therefore, resident pulmonary cells (AM, dendritic cells, epithelial cells, fibroblasts, etc.) and those recruited in inflammatory diseases (lymphocytes, polymorphonuclear cells) express nicotinic receptors.
Nicotinic receptor activation in lymphocytes affects the intracellular signalization, leading to incomplete activation of the cell. In fact, nicotine treatment upregulates protein kinase activity, which in turn upregulates phospholipase A2 (PLA2) activity. PLA2 is responsible for cleaving phosphoinositol-2-phosphate (PIP2) into inositol-3-phosphate (IP3) and diacylglycerol (DAG) (18, 19). The continuous presence of IP3 in the cell would appear to result in the desensitization of calcium stores, leading to their depletion (19). This observation could explain the fact that nicotine-treated lymphocytes do not release enough calcium into the cytoplasm to activate transcription factors such as NFk-B (20).
Nicotine, the major pharmacological component of cigarette smoke, is one of the best known nicotinic receptor agonists (21). This natural substance has well defined anti-inflammatory and immunosuppressive properties (22), and may have anti-fibrotic properties (23). Exposure of animals to smoke from cigarettes with high levels of nicotine is more immunosuppressive than that from low-nicotine cigarettes (24). Moreover, treatment of rats with nicotine inhibits the specific antibody response to antigens and induces T cell anergy (25). Although they are increased in number, AM from smokers show a decreased ability to secrete inflammatory cytokines in response to endotoxins ((20, 25, 26)) and nicotine seems to be the responsible component of this inhibition (26). One study also showed that peripheral blood lymphocytes from smokers express higher levels of FAS ligand (FASL) and that nicotine increases FASL expression on lymphocytes from non-smokers, indicating that nicotine may affect cell apoptosis (27). Nicotine was also shown to have an inhibitory effect on the proliferation and extracellular matrix production of human gingival fibroblasts in vitro (23). Of interest, nicotine treatment seems to up-regulate the expression of nicotinic receptors (28).
Nicotinic agonists may down-regulate T cell activation, indeed, nicotine has been shown to affect T cell expression of the co-stimulatory molecules CD28 and CTLA4 (29).
The B7/CD28/CTLA4 co-stimulatory pathway plays a key regulatory role in T-cell activation and homeostasis (30, 31). Two signaling pathways are involved. A positive signal involves the engagement of B7 (CD80/CD86) molecules with T cell CD28 receptors which results in the potentiation of T cell responses (proliferation, activation, cytokine expression, and survival) (32). A negative signal involves B7 interactions with CTLA4 on activated T cells, leading to a downmodulation of T cell responses (33, 34). The balance between CD28 and CTLA4 derived signals may alter the outcome of T-cell activation.
In HP, it was previously reported that an upregulation of B7 molecule expression on AM in patients with active HP (35) and in murine HP (36). It was also shown that a blockade of the B7-CD28 co-stimulatory pathway in mice inhibited lung inflammation (36). These results also demonstrated that the expression of B7 molecules on AM is lower in smokers than in non-smokers and that an in vitro influenza virus infection is able to upregulate B7 expression in normal human AM but not in AM from smokers; whether this is due to nicotine or other substances present in cigarette smoke is unknown (35). An up-regulation of the B7 molecules has also been reported in asthma (37, 38) and sarcoidosis (39).
Epibatidine is the most potent nicotinic agonist known so far (40). It has anti-inflammatory and analgesic properties. In fact, its analgesic potential is two hundred times that of morphine (40). This molecule is also known to inhibit lymphocyte proliferation in vitro (41). The binding of epibatidine to the receptor is non-specific (42). Unfortunately, epibatidine has major toxic side effects mostly on the cardiovascular and the central nervous systems making it inappropriate for use as an anti-inflammatory drug to treat pulmonary diseases (40).
Dimethylphenylpiperazinium (DMPP) is a synthetic nicotinic agonist that is non-specific (13). Its potency for the receptor is about the same as nicotine, depending on the kind of cells implicated in the stimulation (43). Its advantage over nicotine and other nicotinic agonists is that its chemical configuration prevents it from crossing the blood-brain barrier, thus causing no addiction or other central nervous effects (13). The anti-inflammatory properties of DMPP are not well described. However, it has been shown that a chronic in vivo treatment could decrease the number of white blood cells, decrease the cytokine production by splenocytes and decrease the activity of natural killer cells (44). The effect of DMPP on airway smooth muscle cells has also been tested. DMPP has an initial short contractive effect which is followed by a relaxing effect when the cells are in contact with the agonist for a longer period of time (45). This bronchodilatory effect would not in itself make DMPP a potentially useful treatment of asthma, since more potent bronchodilators are currently available on the market (B2 agonists). However, the properties of this nicotinic receptor agonist are important since this drug could be safely administered to asthmatics and COPD patients for its anti-inflammatories properties. Moreover, there is no evidence that DMPP has any toxic effect on major organs such as the heart, the brain, the liver or the lungs.
Despite advances in the treatment of inflammatory illnesses, including pulmonary inflammatory diseases, treatment using available drugs or agents frequently results in undesirable side effects. For example, the inflammation of COPD is apparently resistant to corticosteroids, and consequently the need for the development of new anti-inflammatory drugs to treat this condition has been recognized (46).
Similarly, while corticosteroids and other immunosuppressive medications have been routinely employed to treat pulmonary fibrosis, they have demonstrated only marginal efficacy (47).
There is thus a need for new and reliable methods of treating inflammatory diseases, including pulmonary inflammatory diseases, in a manner that alleviates their symptoms without causing side effects.